Various proposals have been made in the art for achieving effective marking of substrate, by causing a change of color in or on the substrate, on which the marking, image or print is to appear. In the art, various irreversible thermochromic marking components, i.e. activatable pigments, are disclosed. Substrates coated with such irreversible thermochromic marking components may be colored by application of heat, such as by lasers; thereby the coated substrate may be marked. Marking of a substrate with a laser or some other irradiation source, may be also denoted printing or imaging, as the laser and/or the substrate typically is moved during the process of marking the substrate.
As an example, WO 02/01250 discloses the use of various oxymetal salts, such as ammonium octamolybdate (AOM), as marking component in laser marking. AOM is an example of a marking component that may be marked directly with 10,600 nm laser radiation. Laser radiation having a wavelength of about 10 μm may for example be obtained by use of mid-IR CO2 lasers with an emission wavelength in the range 10,000 nm to 12,000 nm. However, mid-IR CO2 lasers are less suitable for installation into existing production lines due to their physical bulk size. Thus, mid-IR CO2 lasers are less suited for marking of substrates to be marked in a existing production line, such as a production line for liquid food packages.
An NIR fiber laser may have a small print-head fitted to the production line, being connected to the laser, several meters away, via an umbilical cord. Thus, the disadvantage or mid-IR CO2 lasers, being less suitable for installation into existing production lines due to their physical bulk size, may be overcome by use of a NIR (near infra-red) laser and by addition of a NIR-absorber to the ink formulation. The NIR-absorber will absorb near infra-red laser irradiation and convert it into conductive heat. Thus, ink formulations comprising a marking component, e.g. AOM, and a NIR-absorber may be marked by use of a NIR-laser instead of a mid-IR laser. A common example of a NIR-laser is Nd:YAG laser.
Leuco dyes are another type of marking components. They are colorless in one form, but may change into a colored form upon exposure to a particular stimulus. Most leuco dyes are halochromic, i.e. they respond to changes in pH, typically being colorless under alkaline conditions, but being colored in an acidic environment. The color change of halochromic leuco dye may occur when the leuco dye is protonated. Such protonation may result in changes in the conjugate systems of the leuco dye, thereby a conjugated system may be form, having ability to absorb photons of visible light, and therefore appearing colored.
By combining a halochromic leuco dye and a thermal acid generator (TAG) releasing protons upon heating, also halochromic leuco dye may be used in thermal printing, such as in laser printing. A well known and effective TAG is benzylhydroxybenzoate, which in combination with various halochromic leuco dyes, provides ink formulation having god printability.
Charrables are another type of marking components. A charrable is a substance that upon heating undergoes a charring reaction to yield contrasting dark brown/black colour. Typically charrable compounds comprise carbon, hydrogen and oxygen and upon heating undergo a series of reactions that result in the loss of hydrogen and oxygen. Examples of charrable compounds include amino acids, carbohydrates and saccharides. Typical charrable compounds are sugars and polysugars, such as glucose, maltodextrin and saccharose. Commonly, the charrable agent is used in combination with an acidic or basic compound, such as acidic or basic salt. Examples include ammonium salts such as ammonium phosphates and sodium carbonate and sodium bicarbonate. Laser marking based on charring reactions usually require a fairly high fluence >4 J/cm2 in order to mark with sufficient contrasting color. Further, use of charrable compounds are especially interesting for use in applications in connection with food stuff, as charrables such as glucose, maltodextrin and saccharose, are non-toxic and often used already present in food stuff.
In certain applications there is a need for covering the substrate with a thermoplastic polymer layer. By covering the substrate with a thermoplastic polymer layer, certain properties of the substrate, such as the impermeability to liquids, are improved. Further, a thermoplastic polymer layer may also be used to protect an underlying print. For packaging material, typically being a laminate including at least one layer of paper, cardboard, corrugated paper board, or paper board, the outermost layer, or one of the outer layers covering, the substrate is typically low-density polyethylene (LDPE)) or polypropylene melt extruded onto the laminate. For such polyolefins, a temperature of 200 to 340° C. is typically used in melt extruding the polyolefin.
As already described, marking components in the art are typically thermo activated. Typically, discoloration is thus seen, due to activation of the marking component, if such marking components are over laminated by melt extrusion. Accordingly, heat activated marking components are thus presently coated on top of the outermost layer of packaging material laminate, as said layer typically is a melt extruded polyolefin layer, in order to avoid discoloration prior to printing. However, also such materials suffers from disadvantages, as the marking of such coating by applying heat, typically by laser irradiation, may damage the thermoplastic polymer layer.
In order to protect the ink formulation, it would be desirable to have a markable ink formulation less prone to discoloration during over lamination by melt extrusion. Further, it would be desirable to be able to mark a substrate having an ink formulation covered by a thermoplastic polymer without damaging the thermoplastic polymer layer.